This invention concerns computer controlled grinding machines, especially such machines which are to be used for grinding workpieces requiring small diameter grinding wheels to be employed. By small diameter is meant wheels of 200 mm diameter or less.
It is known to mount more than one grinding wheel on a grinding machine, but by arranging two wheels in accordance with the invention unexpected benefits have been found to follow.
According to the present invention, a grinding machine comprises a main frame, a grinding wheel support, and a worktable defining a workpiece axis, wherein the wheel support is slidable relative to the mainframe perpendicularly to the workpiece axis, and the worktable is slidable relative to the main frame perpendicularly to the direction of movement of the wheel support; and a computer supplied with data indicative of at least one operational parameter of the grinding process; wherein the wheel support feed is under the control of signals generated by the computer, and wherein
(a) a frame is hingably mounted on the wheel support parallel to the workpiece axis,
(b) two independently driven small diameter grinding wheels are mounted on the frame remote from the hinged mounting,
(c) pivoting of the frame causes the axis of one or the other of the two small wheels to be aligned with the workpiece axis,
(d) the worktable includes a headstock including a workpiece drive for rotating the workpiece during grinding, and
(e) the pivoting of the frame and the speed of rotation of the workpiece drive are also controlled by signals generated by the computer.
Preferably each grinding wheel is mounted at one end of a spindle which may include one or more hydrostatic bearings and the central shaft of each spindle is directly driven by a motor, at the other end of the spindle.
Where the worktable includes a tailstock preferably the length of each spindle is such as to position the motors axially clear of the tailstock assembly when the wheels are aligned to engage regions of a workpiece nearest to the headstock.
By using relatively small diameter motors and spindles, relatively small diameter wheels can be utilised, which has considerable advantages.
In one arrangement, one of the wheels can be utilised for rough grinding and the other for finish grinding a workpiece, and the wheels are selected accordingly.
A preferred form of this one arrangement is such that the upper grinding wheel is arranged to rough grind, and the lower wheel is arranged to finish grind, the workpieces. In this way the rough grinding process is carried out with the frame in its lowered position, in which the overall assembly of frame, wheelsupport and machine frame is potentially stiffer than when the frame is in its raised condition.
In another arrangement, the wheels are similar and both perform the same grinding function, and one wheel and then the other is used in turn, so that wheel wear is evenly spread between the two wheels, and grinding only has to be interrupted for replacing worn wheels after both wheels have been worn down to an unacceptable level. Since there are two wheels to wear, the time period between machine down times for replacing wheels, is approximately twice the period that would apply if only one wheel were employed.
The hinging of the frame relative to the wheelsupport allows the axis of either of the two grinding wheels to be aligned with the workpiece axis simply by lifting or lowering the frame relative to the wheelsupport.
Pivoting of the frame about its hingable connection to the wheelsupport, may be performed using a pneumatic, hydraulic or electric drive.
Preferably the drive for advancing and retracting the wheelsupport is a linear drive, such as a linear electromagnetic drive, and hydrostatic bearings are provided to support the wheelsupport on a slideway which itself comprises part of the linear drive.
In a preferred embodiment of the invention, a grinding machine comprises a main frame, a grinding wheelsupport, a worktable, a headstock and tailstock carried by the worktable and defining a workpiece axis, wherein the wheelsupport is slidable perpendicularly to the workpiece axis, the tailstock is slidably adjustable relative to the headstock along a slideway carried by the worktable, the latter is slidable relative to the main frame on which a slideway for the wheelsupport is also mounted, and the sliding movement of the worktable relative to the main fame is perpendicular to the direction of movement of the wheelsupport along its slideway, a computer supplied with data indicative of at least one operational parameter of the grinding process, and the wheelfeed is under the control of signals generated by the computer, wherein two independently driven small diameter grinding wheels are mounted on spindles mounted at the outboard end of a frame which is pivotally joined to the wheelsupport, the headstock includes a workpiece drive for rotating the workpiece during grinding, and the speed of rotation of the workpiece drive is also controlled by the signals generated by the computer.
Where the component to be ground is cylindrical, gauging means may be mounted to the grinding machine to enable gauging to be performed, without demounting the workpiece.
Preferably a supply of fluid coolant is provided with means for selectively supplying coolant fluid at an adjustable flow rate, towards the wheel is being employed to grind at the time.
When grinding cylindrical components, uneven wear of grinding wheels, especially CBN wheels, means that sparkout will not necessarily result in a cylindrically true component, and bounce and chatter marks have regularly been found after sparkout is completed. This has been particularly noted when using CBN wheels to grind steel components, such as crankpins of steel crankshafts.
The roundness and surface errors seem to be aggravated when using CBN wheels where separation forces are far higher than for example when using A10x grinding wheels. The stiffness of a CBN wheel is higher than that of an A10x wheel of similar size, and the amount of deflection produced when using a CBN wheel tends to be greater than when using an A10x wheel. These deflections, coupled with the hydrodynamic effect of high pressure coolant, have meant that during sparkout the grinding wheel has tended to bounce into and out of contact with the surface being ground. Chatter marks induced by this bounce seem to be worse when the surface being ground is rotating away from the grinding wheel (ie when the part is not being forced/rotated onto the wheel).
It has been found desirable that u hen grinding workpieces such as crankpins of crankshafts using a two-wheel grinding machine as aforesaid having a computer controlled wheelfeed and workpiece drive in during the grinding of each pin:
(i) the cutting force is maintained on the wheelhead to keep the wheel and pin under a moderate constant load, even during what would have been the sparkout step of known methods, and
(ii) during at least a final revolution of the workpiece its rotational speed is reduced;
to prevent bounce and therefore chatter marks appearing in the surface of the pin.
Typically the rotational speed of the headstock drive is reduced to a speed in the range 1 to 5 rpm.
Typically the rotational speed is reduced when the depth of metal left to grind is such that it can be removed during a single revolution of the workpiece at the reduced speed, without exceeding the available power in the wheelspindle drive.
Preferably the pin is gauged before the final grinding step is performed, so as to determine the depth of cut which is necessary to achieve finish size, and the wheelfeed is controlled so as to remove that depth to achieve finish size.
Preferably the coolant rate is reduced during the final single revolution of the workpiece, so that whereas the cutting forces remain constant throughout the final grind revolution, the hydrodynamic forces are reduced.
If any roundness errors on a workpiece (eg crankpin) are still found to exist, a computer based component-profile editing procedure may be employed to remove any such errors where the workpieces are similar, since in general these residual errors will tend to be the same and will appear on each pin on every crankshaft ground, of a batch of similar crankshafts.
Thus in one example of the invention, the majority of the metal to be removed to grind a steel crankpin to size using a CBN wheel, is removable in the traditional manner, and as the pin approaches finish size and only approximately 50 um is left on the radius to be removed, the pin is gauged and the precise oversize determined, the workspeed is decreased to say 3 rpm, the coolant supply is reduced and the wheelhead is controlled so as to remove a final depth increment, the size of which is determined by the gauging from around the pin, during a single revolution of the crankshaft, after which the wheelsupport is retracted so that the wheel disengages completely from the pin, without a sparkout step, leaving the pin ground to size.
It has been found that a wheel can become worn in some places more than others around its circumference. This seems to arise due to any out of balance of the wheel. This imbalance is believed to set up a vibration at a particular frequency, causing spaced apart regions around the wheel to wear more than others, so as to produce what is described as a lobe effect on the grinding wheel. This in turn has been found to be one of the causes of regenerative chatter.
According therefore to another aspect of the invention, in a method of grinding cylindrical workpieces such as crankpins of a crankshaft, particularly when using a CBN wheel, the wheel speed of rotation is varied at intervals during the grinding of the workpieces so as to reduce the uneven wear pattern which can otherwise occur around the grinding surface of the wheel.
According to this aspect of the invention, the wheel speed may be changed after every nth pin has been ground.
Typically n equals 3, but can be any value from 1 upwards. Typically the rotational speed change is of the order of xc2x12-5% of the nominal wheel speed.
By changing the wheel speed, so the positions of points at which wear can occur as aforesaid will alter so that any extra wear on the grinding wheel will occur at different places around the circumference of the wheel, instead of always in the same places, during each revolution of the wheel.
In any method as aforesaid for grinding cylindrical components such as crankpins of crankshafts, a gauge may be used to measure the component when the latter is expected to be say 100 m above finish size; and a computer supplied with the gauged size is programmed to correct at least the wheel feed to correct for any difference detected by the gauging between the diameter of the pin at the nominal oversize stage and the diameter expected at that point in the grinding.
Although described in relation to the grinding of cylindrical components, a grinding machine having two small diameter grinding wheels as aforesaid may also be used to grind non-cylindrical components such as cams on a camshaft, especially cams having concave regions on their flanks.
In apparatus and methods as aforesaid the two small grinding wheels will normally have the same nominal diameter.